RECONSTRUCTING THE EXTENT AND TIMING OF THE LAST GLACIATION IN TENSLEEP CANYON, BIGHORN MOUNTAINS, WYOMING, U.S.A

During the Last Glacial Maximum, the Bighorn Mountains of Wyoming and Montana were subject to extensive glaciation by alpine glaciers. Tensleep Canyon held one of the largest glaciers in the range, which produced well-preserved and nearly continuous lateral and terminal moraines. Numerical age limits on the timing of deglaciation in the Bighorn Mountains are limited, however, especially compared to the nearby Yellowstone glacial system. This study presents ¹⁰Be cosmogenic surface-exposure ages from a terminal moraine in Tensleep Canyon to set limits on the timing of moraine deposition in the Bighorn Mountains relative to the Last Glacial Maximum, and to contribute to the growing understanding of this glacial period throughout the Rocky Mountain region. Although cosmogenic ¹⁰Be exposure ages of boulders atop the terminal moraine vary considerably, a cluster of exposure ages has a mean age of 17.3 ± 0.8 kyr. This age limit suggests that the Tensleep Canyon glacier began retreating from its terminal moraine near or after the end the Last Glacial Maximum, similar to age limits on the start of deglaciation from elsewhere in the middle Rocky Mountains. Coincident with deposition of the terminal moraine in Tensleep Canyon was construction of a 16-km-long, continuous right lateral moraine. Along the length of this moraine, basal shear stress estimates for the Tensleep Canyon glacier range from ~25-87 kPa, with a decrease occurring along a segment approximately 4.5 to 9.0 km upvalley of the terminal moraine. Ice movement throughout most of the canyon likely occurred by deformation and basal sliding, with ice and debris fluxes comparable to those in other steep, glaciated canyons of the Rocky Mountains.